Fix __getcwd rewinddir namespace (bug 17584).

Fix __getcwd rewinddir namespace (bug 17584).

__getcwd is called from dcigettext.o (brought in by various ISO C
functionality), but calls rewinddir, which is not an ISO C function.
This patch makes __getcwd call __rewinddir instead and makes rewinddir
a weak alias for __rewinddir.

Since getcwd.c is shared with gnulib (albeit not merged in either
direction for a long time, and omitted from gnulib's
config/srclist.txt list of shared files) I put in a #ifndef _LIBC
define of __rewinddir to rewinddir, although a future merged version
of getcwd could end up looking significantly different.

Tested for x86_64 (testsuite, and that disassembly of installed shared
libraries is unchanged by this patch).

	[BZ #17584]
	* dirent/rewinddir.c (rewinddir): Rename to __rewinddir and define
	as weak alias of __rewinddir.  Don't use libc_hidden_def.
	(__rewinddir): Use libc_hidden_def.
	* sysdeps/mach/hurd/rewinddir.c: Rename to __rewinddir and define
	as weak alias of __rewinddir.  Don't use libc_hidden_def.
	(__rewinddir): Use libc_hidden_def.
	* sysdeps/posix/rewinddir.c: Rename to __rewinddir and define as
	weak alias of __rewinddir.  Don't use libc_hidden_def.
	(__rewinddir): Use libc_hidden_def.
	* include/dirent.h (rewinddir): Don't use libc_hidden_proto.
	(__rewinddir): Use libc_hidden_proto.
	* sysdeps/posix/getcwd.c [!_LIBC] (__rewinddir): Define to
	rewinddir.
	(__getcwd): Use __rewinddir instead of rewinddir.